Recent works have proved that semiclassical theories of gravity needed not be fundamentally inconsistent, at least in the Newtonian regime. Using the machinery of continuous measurement theory and feedback, it was shown that one could construct well-behaved models of hybrid quantum-classical dynamics at the price of an imposed (nonunique) decoherence structure. We introduce a principle of least decoherence (PLD) which allows us to naturally single out a unique model from all the available options; up to some unspecified short distance regularization scale. Interestingly, the resulting model is found to coincide with the old—erstwhile only heuristically motivated—proposal of Penrose and one of us for gravity-related spontaneous decoherence and collapse. Finally, this paper suggests that it is in the submillimeter behavior of gravity that new phenomena might be found.

Lorentz-violating gravity theories with a preferred foliation can have instantaneous propagation. Nonetheless, it has been shown that black holes can still exist in such theories and the relevant notion of an event horizon has been dubbed “universal horizon.” In stationary spacetimes the universal horizon has to reside in a region of spacetime where the Killing vector associated with stationarity is spacelike. This raises the question of what happens to the universal horizon in the extremal limit, where no such region exists anymore. We use a decoupling limit approximation to study this problem. Our results suggest that at the extremal limit, the extremal Killing horizon appears to play the role of a degenerate universal horizon, despite being a null and not a spacelike surface, and hence not a leaf of the preferred foliation.

Given the tension between the values of the Hubble parameter $H_0$ inferred from the cosmic microwave background (CMB) and from supernovae, attention is turning to time delays of strongly lensed quasars. Current time-delay measurements indicate a value of $H_0$ closer to that from supernovae, with errors on the order of a few percent, and future measurements aim to bring the errors down to the subpercent level. Here we consider the uncertainties in the mass distribution in the outskirts of the lens. We show that these can lead to errors in the inferred $H_0$ on the order of a percent and, once accounted for, would correct $H_0$ upward (thus increasing slightly the tension with the CMB). Weak gravitational lensing and simulations may help to reduce these uncertainties.

We numerically study the interactions of twisted monopole-antimonopole pairs in the ’t Hooft-Polyakov model for a range of values of the scalar to vector mass ratio. We also recover the sphaleron solution at maximum twist discovered by Taubes [Commun. Math. Phys. 86, 257 (1982)] and map out its energy and size as functions of parameters.

We study how the presence of world-sheet currents affects the evolution of cosmic string networks, and their impact on predictions for the cosmic microwave background (CMB) anisotropies generated by these networks. We provide a general description of string networks with currents and explicitly investigate in detail two physically motivated examples: wiggly and superconducting cosmic string networks. By using a modified version of the CMBact code, we show quantitatively how the relevant network parameters in both of these cases influence the predicted CMB signal. Our analysis suggests that previous studies have overestimated the amplitude of the anisotropies for wiggly strings. For superconducting strings the amplitude of the anisotropies depends on parameters which presently are not well known—but which can be measured in future high-resolution numerical simulations.

Cosmic-ray antideuterium and antihelium have long been suggested as probes of dark matter, as their secondary astrophysical production was thought extremely scarce. But how does one actually predict the secondary flux? Antinuclei are dominantly produced in $pp$ collisions, where laboratory cross section data are lacking. We make a new attempt at tackling this problem by appealing to a scaling law of nuclear coalescence with the physical volume of the hadronic emission region. The same volume is probed by Hanbury Brown–Twiss (HBT) two-particle correlations. We demonstrate the consistency of the scaling law with systems ranging from central and off-axis $AA$ collisions to $pA$ collisions, spanning 3 orders of magnitude in coalescence yield. Extending the volume scaling to the $pp$ system, HBT data allow us to make a new estimate of coalescence that we test against preliminary ALICE $pp$ data. For antihelium the resulting cross section is 1 or 2 orders of magnitude higher than most earlier estimates. The astrophysical secondary flux of antihelium could be within reach of a five-year exposure of AMS02.

We investigate compact halos of sterile-neutrino dark matter and examine observable signatures with respect to neutrino and photon emission. Primarily, we consider two cases: primordial black-hole halos and ultracompact minihalos. In both cases, we find that there exists a broad range of possible parameter choices such that detection in the near future with x-ray and gamma-ray telescopes might be well possible. In fact, for energies above 10 TeV, the neutrino telescope IceCube would be a splendid detection machine for such macroscopic dark-matter candidates.

Unresolved sources of gravitational waves are at the origin of a stochastic gravitational wave background. While the computation of its mean density as a function of frequency in a homogeneous and isotropic universe is standard lore, the computation of its anisotropies requires understanding the coarse graining from local systems, to galactic scales and then to cosmology. An expression of the gravitational wave energy density valid in any general spacetime is derived. It is then specialized to a perturbed Friedmann-Lemaître spacetime in order to determine the angular power spectrum of this stochastic background as well as its correlation with other cosmological probes, such as the galaxy number counts and weak lensing. Our result for the angular power spectrum also provides an expression for the variance of the gravitational wave background.

We report here the results of searching for inelastic scattering of dark matter (initial and final state dark matter particles differ by a small mass splitting) with a nucleon for the first 79.6 days of PandaX-II data (Run 9). We set the upper limits for the spin independent weakly interactive massive particle–nucleon scattering cross section up to a mass splitting of $300\mathrm{keV}/{\mathrm{c}}^2$ at two benchmark dark matter masses of 1 and $10\mathrm{TeV}/{\mathrm{c}}^2$.

Strongly interacting matter undergoes a crossover phase transition at high temperatures $T\sim {10}^{12}\mathrm{K}$ and zero net-baryon density. A fundamental question in the theory of strong interactions, QCD, is whether a hot and dense system of quarks and gluons displays critical phenomena when doped with more quarks than antiquarks, where net-baryon number fluctuations diverge. Recent lattice QCD work indicates that such a critical point can only occur in the baryon dense regime of the theory, which defies a description from first principles calculations. Here we use the holographic gauge/gravity correspondence to map the fluctuations of baryon charge in the dense quark-gluon liquid onto a numerically tractable gravitational problem involving the charge fluctuations of holographic black holes. This approach quantitatively reproduces ab initio results for the lowest order moments of the baryon fluctuations and makes predictions for the higher-order baryon susceptibilities and also for the location of the critical point, which is found to be within the reach of heavy-ion collision experiments.

We present estimates of transverse single spin asymmetry (TSSA) in $p+p^{\uparrow }\to J/\psi +X$ within the color evaporation model of charmonium production in a generalized parton model (GPM) framework, using the recently obtained best fit parameters for the gluon Sivers function (GSF) extracted from PHENIX data on TSSA in $p+p^{\uparrow }\to {\pi }^0+X$ at midrapidity. We calculate asymmetry at $\sqrt{s}=200\mathrm{GeV}$, and compare the results with PHENIX data on TSSA in the process $p+p^{\uparrow }\to J/\psi +X$. We also present estimates for asymmetry at $\sqrt{s}=115\mathrm{GeV}$ corresponding to the proposed fixed target experiment AFTER@LHC and at $\sqrt{s}=500\mathrm{GeV}$ corresponding to the higher RHIC energy. Finally, we investigate the effect of the transverse momentum dependent (TMD) evolution of the densities involved, on the asymmetry.

We build the electromagnetic trace anomaly effective term for a generalized linear sigma model with two chiral nonets, one with a quark-antiquark structure and the other one with a four-quark content. In the leading order of this framework, we study the decays into two photons of the lowest isosinglet scalar mesons. We find that the direct inclusion of underlying mixing among two- and four-quark components in the trace anomaly term is essential in order for the model prediction to agree with the available experimental data on the decay width of $f_0(980)$ to two photons. Consequently, this sets a lower bound of 0.5 KeV on the decay with of $f_0(500)$ to two photons.

We study an interesting region of phase space at the LHC for pair-produced stops decaying into hadronic top quarks and light neutralinos. After imposing a sizeable cut on the missing transverse energy, which is the key variable for reducing backgrounds, we find that the two hadronic tops are likely to merge into a single fat jet. We develop a jet-substructure-based strategy to tag the two merged top jets and utilize the $M_{T2}$ variable to further reduce the backgrounds. We obtain about a 40% increase to the ratio of the signal over background and a mild increase on the signal discovery significance, based on a signal with a 1.2 TeV stop and a 100 GeV neutralino, for the 13 TeV LHC with $100{\mathrm{fb}}^{-1}$. The general event kinematics could also occur and be explored for other new physics signatures with large missing transverse energy.

The LHCb measurements of certain ratios of decay modes testing lepton flavor nonuniversality might open an exciting world of new physics beyond the standard model. The latest LHCb measurements of $R_{K^{*}}$ offer some new insight beyond the previous measurement of $R_K$. We work out the present significance for nonuniversality, and argue that claims of $5\sigma$ deviations from the standard model based on all present $b\to s{\ell }^{+}{\ell }^{-}$ data including the ratios are misleading and are at present still based on guesstimates of hadronic power corrections in the $b\to s{\ell }^{+}{\ell }^{-}$ angular observables. We demonstrate that only a small part of the luminosity of $50{\mathrm{fb}}^{-1}$ foreseen to be accumulated by the LHCb will be needed to offer soon a definite answer to the present question of whether we see a very small glimpse of lepton flavor nonuniversal new physics or not. We also present new predictions for other ratios based on our analysis of the present measurements of the ratios $R_{K^{(*)}}$ and analyze if they are able to differentiate between various new physics options within the effective field theory at present or in the near future.

The LHCb has measured the ratios of $B\to K^{*}{\mu }^{+}{\mu }^{-}$ to $B\to K^{*}{e}^{+}{e}^{-}$ branching fractions in two dilepton invariant mass squared bins, which deviate from the standard model predictions by approximately $2.5\sigma$. These new measurements strengthen the hint of lepton flavor universality breaking which was observed earlier in $B\to K{\ell }^{+}{\ell }^{-}$ decays. In this work we explore the possibility of explaining these anomalies within the framework of $R$-parity violating interactions. In this framework, $b\to s{\ell }^{+}{\ell }^{-}$ transitions are generated through tree and one loop diagrams involving exchange of down-type right-handed squarks, up-type left-handed squarks and left-handed sneutrinos. We find that the tree level contributions are not enough to explain the anomalies, but at one loop, simultaneous explanation of the deviations in $B\to K^{*}{\ell }^{+}{\ell }^{-}$ and $B\to K{\ell }^{+}{\ell }^{-}$ is feasible for a parameter space of the Yukawa couplings that is consistent with the bounds coming from $B\to K^{(*)}\nu \overline{\nu }$ and $D^0\to {\mu }^{+}{\mu }^{-}$ decays and $B_s-{\overline{B}}_s$ mixing.

We consider effective Higgs boson couplings, including both $CP$-even and $CP$-odd couplings, which affect Higgs boson pair production. Through partial wave analysis, we find that the process $gg\to hh$ is dominated by the $s$-wave component even at a 100 TeV $pp$ collider. Making use of $s$-wave kinematics, we propose a cut efficiency function to mimic the collider simulation and obtain the potential of measuring Higgs effective couplings at the 14 TeV LHC with an integrated luminosity of $3000{\mathrm{fb}}^{-1}$ and at a 100 TeV $pp$ collider. Analytical expressions of 2σ exclusion limits at the LHC and $5\sigma$ discovery bands at the 100 TeV machine are given.

We continue the study of the nonrelativistic short-distance completions of a naturally light Higgs, focusing on the interplay between the gauge symmetries and the polynomial shift symmetries. We investigate the naturalness of nonrelativistic scalar quantum electrodynamics with a dynamical critical exponent $z=3$ by computing leading power law divergences to the scalar propagator in this theory. We find that power law divergences exhibit a more refined structure in theories that lack boost symmetries. Finally, in this toy model, we show that it is possible to preserve a fairly large hierarchy between the scalar mass and the high-energy naturalness scale across 7 orders of magnitude, while accommodating a gauge coupling of order 0.1.

The role of gluons in the structure of the nucleon and light nuclei is investigated using lattice quantum chromodynamics (QCD) calculations. The first moment of the unpolarized gluon distribution is studied in nuclei up to atomic number $A=3$ at quark masses corresponding to pion masses of $m_{\pi }\sim 450$ and 806 MeV. Nuclear modification of this quantity defines a gluonic analogue of the EMC effect and is constrained to be less than $\sim 10\%$ in these nuclei. This is consistent with expectations from phenomenological quark distributions and the momentum sum rule. In the deuteron, the combination of gluon distributions corresponding to the $b_1$ structure function is found to have a small first moment compared with the corresponding momentum fraction. The first moment of the gluon transversity structure function is also investigated in the spin-1 deuteron, where a nonzero signal is observed at $m_{\pi }\sim 806\mathrm{MeV}$. This is the first indication of gluon contributions to nuclear structure that can not be associated with an individual nucleon.

A possible method to solve the sign problem is developed by modifying the original theory. Considering several modifications of the partition function, the observable in the original theory is reconstructed from the identity connecting the observables in the original and modified theories. We demonstrate that our method gives the correct results, even if the original theory has the severe sign problem, by using a simple one-dimensional integral.

We give heavy quark-diquark symmetry predictions for doubly heavy baryons and tetraquarks in light of the recent discovery of the ${\mathrm{\Xi }}_{cc}^{++}$ by LHCb. For five excited doubly charm baryons that are predicted to lie below the ${\mathrm{\Lambda }}_{c}D$ threshold, we give predictions for their electromagnetic and strong decays using a previously developed chiral Lagrangian with heavy quark-diquark symmetry. Based on the mass of the ${\mathrm{\Xi }}_{cc}^{++}$, the existence of a doubly heavy bottom $I=0$ tetraquark that is stable to strong and electromagnetic decays has been predicted. If the mass of this state is below 10405 MeV, as predicted in some models, we argue using heavy quark-diquark symmetry that the $J^P=1^{+}I=1$ doubly bottom tetraquark state will lie just below the open bottom threshold and likely be a narrow state as well. In this scenario, we compute strong decay width for this state using a new Lagrangian for doubly heavy tetraquarks which is related to the singly heavy baryon Lagrangian by heavy quark-diquark symmetry.